Plant cell walls are essential for proper growth, development, and interaction with the environment. It is generally accepted that land plants arose from aquatic ancestors which are sister groups to the charophycean algae (i.e., Streptophyta), and study of wall evolution during this transition promises insight into structure-function relationships of wall components. In this paper, we explore wall evolutionary history by studying the incorporation of pectin polymers into cell walls of the model organism Penium margaritaceum, a simple single-cell desmid. This organism produces only a primary wall consisting of three fibrillar or fibrous layers, with the outermost stratum terminating in distinct, calcified projections. Extraction of isolated cell walls with trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid yielded a homogalacturonan (HGA) that was partially methyl esterified and equivalent to that found in land plants. Other pectins common to land plants were not detected, although selected components of some of these polymers were present. Labeling with specific monoclonal antibodies raised against higher-plant HGA epitopes (e.g., JIM5, JIM7, LM7, 2F4, and PAM1) demonstrated that the wall complex and outer layer projections were composed of the HGA which was significantly calcium complexed. JIM5 and JIM7 labeling suggested that highly methyl esterified HGA was secreted into the isthmus zone of dividing cells, the site of active wall secretion. As the HGA was displaced to more polar regions, de-esterification in a non-blockwise fashion occurred. This, in turn, allowed for calcium binding and the formation of the rigid outer wall layer. The patterning of HGA deposition provides interesting insights into the complex process of pectin involvement in the development of the plant cell wall.

At early stages of Arabidopsis (Arabidopsis thaliana) flowering, the inflorescence stem undergoes rapid growth, with elongation occurring predominantly in the apical ;4 cm of the stem. We measured the spatial gradients for elongation rate, osmotic pressure, cell wall thickness, and wall mechanical compliances and coupled these macroscopic measurements with molecular-level characterization of the polysaccharide composition, mobility, hydration, and intermolecular interactions of the inflorescence cell wall using solid-state nuclear magnetic resonance spectroscopy and small-angle neutron scattering. Force-extension curves revealed a gradient, from high to low, in the plastic and elastic compliances of cell walls along the elongation zone, but plots of growth rate versus wall compliances were strikingly nonlinear. Neutron-scattering curves showed only subtle changes in wall structure, including a slight increase in cellulose microfibril alignment along the growing stem. In contrast, solid-state nuclear magnetic resonance spectra showed substantial decreases in pectin amount, esterification, branching, hydration, and mobility in an apical-to-basal pattern, while the cellulose content increased modestly. These results suggest that pectin structural changes are connected with increases in pectin-cellulose interaction and reductions in wall compliances along the apical-to-basal gradient in growth rate. These pectin structural changes may lessen the ability of the cell wall to undergo stress relaxation and irreversible expansion (e.g. induced by expansins), thus contributing to the growth kinematics of the growing stem.

Physical and functional interactions between pectin and cellulose are required for normal wall assembly and plant growth, as revealed in analyses of mutants for a pectin methyltransferase.

Cell-wall (CW) development in the desmid Penium margaritaceum (Ehrenb.) Bréb. was studied using immunofluorescence labeling of living cells with the monoclonal antibodies (mAbs) JIM5 and JIM7, which recognize unesterified and methyl-esterified homogalacturonan (HG), respectively. During cell expansion, HG was secreted in a high-esterified form at a narrow band, called the HG secretion band or HGSB, at the isthmus or the polar tip of a daughter semicell. As newly secreted HG is displaced outward on the cell surface, deesterification and subsequent calcium (Ca(2+) )-complexing occurred to yield a rigid covering. HG secretion and CW/cell expansion were reversibly inhibited by dark, brefeldin A (BFA), and incubation in 0.24-0.36 M sucrose but were not altered by treatment with actin/microfilament drugs. The HGSB was detected near the nucleus during most cell-cycle events. Centrifugation displaced the nucleus away from the HGSB, but HG synthesis was not affected. HGSB activity was preceded by, and coordinated with, Calcofluor labeling, which suggests that cellulose production in CW/cell-expansion sites was critical to expansion control. In many first-cell-division products, asymmetric patterning of HG was noted in the CW. These asymmetric patterns most likely were a result of timing mechanisms and displacement of the nucleus-HGSB during the cell cycle.

(1,3)(1,4)-β-D-Glucan (mixed-linkage glucan or MLG), a characteristic hemicellulose in primary cell walls of grasses, was investigated to determine both its role in cell walls and its interaction with cellulose and other cell wall polysaccharides in vitro. Binding isotherms showed that MLG adsorption onto microcrystalline cellulose is slow, irreversible, and temperature-dependent. Measurements using quartz crystal microbalance with dissipation monitoring showed that MLG adsorbed irreversibly onto amorphous regenerated cellulose, forming a thick hydrogel. Oligosaccharide profiling using endo-(1,3)(1,4)-β-glucanase indicated that there was no difference in the frequency and distribution of (1,3) and (1,4) links in bound and unbound MLG. The binding of MLG to cellulose was reduced if the cellulose samples were first treated with certain cell wall polysaccharides, such as xyloglucan and glucuronoarabinoxylan. The tethering function of MLG in cell walls was tested by applying endo-(1,3)(1,4)-β-glucanase to wall samples in a constant force extensometer. Cell wall extension was not induced, which indicates that enzyme-accessible MLG does not tether cellulose fibrils into a load-bearing network.